Liquid crystal displays have now surpassed conventional CRT units and become a main stream in the market due to its compact appearance, energy-efficient feature, improved image quality and a wide range of applications.
FIG. 1 schematically shows components of a display area of an LCD panel. In the display area 212, matrices of pixel electrodes 221, thin film transistors (TFT) 222, gate lines 223 and data lines 224 are formed on a lower glass substrate 211. Above the pixel electrodes 221, an alignment film 225 is provided. Oppositely, on almost the entire surface of an upper glass substrate 231 facing to the lower glass substrate 211, a common electrode 233 and an alignment film 232 are formed. Furthermore, a liquid crystal (LC) layer 241 is sealed in the space between the lower alignment film 225 and the upper alignment film 232.
With the alignment films 225 and 232, liquid crystal molecules in the LC layer 241 are specifically and differentially oriented. The orientation of the alignment films is determined depending on the type of the LCD, and varies with the structures and/or material of the alignment films. For example, liquid crystal molecules arrange themselves twisted, e.g. in a helical structure, in a twisted nematic (TN) LCD before an electric field is applied. On the other hand, in a vertical alignment (VA) LCD, liquid crystal molecules naturally arrange themselves vertically. When no voltage is applied, the liquid crystal molecules of a VA LCD remain perpendicular to the substrate so as to render a black display. When a voltage is applied, the liquid crystal molecules change toward a horizontal direction, i.e. a direction parallel to the substrate, thereby allowing light to pass through and creating a white display.
As known, a VA LCD has good contrast when viewed vertically. However, image quality would be adversely affected if viewed at a relatively large viewing angle. FIG. 2 illustrates viewing conditions of a VA LCD at different view positions. As shown, when liquid crystal molecules 33 tilt in response to a voltage, a gray color can be seen at a viewing position A right in front of the display. However, at viewing positions B and C, black and white colors are seen, respectively, due to different tilting angles of liquid crystal molecules 33 relative to different viewing positions. As a result, the displaying is distorted.
For remedying the defect, a multi-domain vertical alignment (MVA) LCD is developed, as illustrated in FIG. 3. As shown, a pixel is divided into a plurality of domains and liquid crystal molecules 33 are oriented differently in different domains, e.g. tilting counterclockwise in the left portion 31 and tilting clockwise in the right portion 32 of the pixel. Accordingly, under the similar condition of the grey color as illustrated in FIG. 2, the left portion 31 of the pixel is shown black while the right portion 32 of the pixel is shown white at the viewing position C. On the other hand, at the viewing position B, the left portion 31 of the pixel is shown white while the right portion 32 of the pixel is shown black. Therefore, the pixel is shown substantially even grey at arbitrary viewing positions. Generally, four domains are proper for wide angle viewing.
However, it is difficult in practice to divide a single pixel which has a size as small as 100×300 μm into four domains and control the liquid crystal molecules in the four domains to be oriented differentially. For facilitating differential orientation of liquid crystal molecules in different domains, a bump structure is provided either between the pixel electrode and its associated alignment film or the common electrode and its associated alignment film or both for automatic domain formation. Please refer to FIG. 4, which illustrates the principle of automatic domain formation with an example.
As shown in the example of FIG. 4, a bump structure 41 is formed between the pixel electrode 40 and the lower alignment film 42 at a boundary 43 of domains. Due to the presence of the bump structure 41, some of the liquid crystal molecules 44 distributed above the bump structure 41 and supposed to stand vertically when no voltage is applied tilt. Then the tilting action propagates as indicated by arrows so that the liquid crystal molecules in the same domain are oriented consistently. Since the configuration of the bump structure makes the liquid crystal molecules distributed above the bump structure 41 tilt in different directions, the liquid crystal molecules in different domains are oriented differentially.
Alternatively or additionally, the bump structure 41 may be provided between the upper alignment film 45 and the common electrode 46.
Although differential orientation of liquid crystal molecules in multi-domains can be achieved by way of bump structures as described above, the formation of the bump structure complicates the manufacturing process of the LCD panel. Therefore, slits are created in either the pixel electrode 50 or the common electrode 52 to replace the bump structure 41 to achieve the object of differential orientation of liquid crystal molecules, as illustrated in FIG. 5A or FIG. 5B. The slits 51 are arranged in the pixel electrode 50 or the common electrode 52. The shape of the slits 51 may be a circle or a “+” cross when viewed from top. In general, the use of the cross-shaped slits results in better transmittance than the use of circular slits. It is to be noted that for neat drawing, alignment films are not particularly shown in the figures.
However, due to the cross-shaped configuration of the slits, it takes a long response time to reach stable LC alignment since complicated movement of liquid crystal molecules is involved, including directing liquid crystal molecules 61a disposed above the slit 60 to a right angle, then directing liquid crystal molecules 61b near the slit 60 to rotate to an oblique angle and then expanding other liquid crystal molecules 61c to the oblique angle, as illustrated in FIG. 6A-6C.